In recent years, development of techniques for selectively separating carbon dioxide in a mixed gas is advancing. For example, as a countermeasure for global warming, a technique of collecting carbon dioxide in exhausted gas and condensing it, and a technique of reforming hydrocarbon into hydrogen and carbon monoxide (CO) by means of steam reforming, further allowing the carbon monoxide to react with steam to form carbon dioxide and hydrogen, and excluding the carbon dioxide by using a membrane which is selectively permeable to carbon dioxide, thereby obtaining gas for fuel cells or the like, which includes hydrogen as a main component, have been developed.
Meanwhile, regarding separation of carbon dioxide, an amine absorption method, in which adsorption and desorption are repeated by amines, is a general method and has been widely used. However, this method is disadvantageous in that a huge installation area is needed for the facilities and, in addition, it is necessary to repeat increasing pressure/decreasing pressure and lowering temperature/elevating temperature at the time of adsorption/desorption, which needs a large amount of energy. Further, the capacity of the system has been determined at the time of planning, and thus, it is not easy to increase or decrease the capacity of the system once formed.
In contrast, a membrane separation method is a method of performing separation naturally by utilizing the partial pressure of carbon dioxide in the two regions separated by a separation membrane and is advantageous in that consumption of energy is low and the installation area is small. Further, increase or decrease in the capacity of the system can be conducted by increasing or decreasing the number of filter units and therefore, it is possible to provide a system having excellent scalability; accordingly, the membrane separation method has recently attracted attention.
Carbon dioxide separation membranes can be roughly classified into so-called accelerated transport membranes, in which a carbon dioxide carrier is included in the membrane and carbon dioxide is transported to the opposite side of the membrane by this carrier, and so-called dissolution diffusion membranes, with which separation is performed by utilizing the difference in solubility with respect to the membrane and the difference in diffusivity in the membrane, between carbon dioxide and the substance to be subject to separation. Since a dissolution diffusion membrane is used to perform separation based on the solubilities of carbon dioxide and the substance to be subject to separation with respect to the membrane and the diffusion speeds, the degree of separation is determined unequivocally when the material and physical properties of the membrane are determined, and further, since the permeation speed increases as the thickness of the membrane gets thinner, the dissolution diffusion membrane is generally produced as a thin membrane having a thickness of 1 μm or less, by using a layer separation method, a surface polymerization method, or the like.
In contrast, in an accelerated transport membrane, by the addition of a carbon dioxide carrier into the membrane, the solubility of carbon dioxide is drastically increased, and transportation is carried out under a high concentration environment. Accordingly, the accelerated transport membrane is characterized in that, in general, the degree of separation with respect to the substance to be separated is higher and the permeation speed of carbon dioxide is higher, as compared with a dissolution diffusion membrane. Further, since the concentration of carbon dioxide in the membrane is high, the diffusion of carbon dioxide in the membrane rarely becomes a rate-limiting factor, and in the sense of increasing the degree of separation with respect to the substance to be separated, it is more preferable that the accelerated transport membrane is a thick membrane having a thickness of 1 μm or more.
For example, Japanese Patent Publication (JP-B) No. H7-102310, a technique for producing a carbon dioxide separation gel membrane has been proposed, the technique including coating an aqueous solution of an uncrosslinked vinyl alcohol-acrylic acid salt copolymer on a carbon dioxide permeable support to form a membrane, then heating and crosslinking the membrane to become water insoluble, and then allowing this water insoluble substance to absorb an aqueous solution containing a carbon dioxide carrier (a substance that has affinity with carbon dioxide), to gelate the membrane.
Further, in Japanese Patent Application Laid-Open (JP-A) No. 2009-195900, a carbon dioxide separation apparatus has been proposed, in which a gel layer obtained by adding an additive composed of cesium carbonate or cesium hydrogencarbonate or cesium hydroxide to a polyvinyl alcohol-polyacrylic acid copolymer gel membrane is provided on a hydrophilic porous membrane to form a CO2 accelerated transport membrane, and a source gas including at least carbon dioxide and steam as well as a certain main component of gas is supplied to the surface of the source side of the CO2 accelerated transport membrane at a supply temperature of 100° C. or higher, and then the carbon dioxide, that has been permeated through the CO2 accelerated transport membrane, is taken out from the surface of the permeation side.